The present invention is generally related to control of electromechanical machines, and, more particularly, the present invention is related to method and system for controlling a permanent magnet (PM) machine during fault conditions.
In the control of inverter-driven PM machines used in propulsion systems, field weakening is often used to lower the inverter current and/or voltage rating for a given application. That is, without field weakening the inverter manufacturer would have to use components rated to handle higher levels of current and/or voltage for that given application. This would undesirably add incremental costs to the propulsion system. Field weakening is generally accomplished by configuring the machine windings to provide a greater torque per amp ratio, and thus achieve a lower base speed for a given torque load. During high speed operation, the phase current may be applied to the machine windings in advance of the phase electromotive force (EMF), and thus the EMF, while greater in peak magnitude than the energy source voltage, would have an apparent magnitude lower that the source voltage.
Above the base speed of the machine, where the line-to-line EMF voltage due to the magnets has become greater or equal to the source voltage, a field-weakening current needs to be applied to the machine in order for torque to be realized. The flux created by this current is in opposition to the magnet""s flux, and this reduces the effective EMF seen by the inverter. It should be stressed that this current should be carefully regulated to a target value set by the commanded torque and rotor speed. Failure to control the current to an appropriate value will likely result in the undesirable scenario that excessive voltage is applied to the inverter.
The foregoing technique works well, except when the energy source, e.g., a battery, is intentionally or unintentionally disconnected and the machine is operating in the field-weakening mode. Under this condition, if corrective action is not taken, damage to the inverter could occur due to excessive voltage across the inverter and the DC bus. This excessive voltage is due to a charging mode overcharging the bus capacitance or a motoring mode depleting the bus capacitance to the point where the current can no longer be regulated. In either case, the EMF of the machine would be impressed on the inverter. If this voltage were to exceed the voltage ratings of the power semiconductor devices, or capacitors or other circuitry used by the system, costly damage to the system may occur.
Thus, in view of the foregoing considerations, it would be desirable to provide a method and system for maintaining an appropriate level of field-weakening current and for controlling the DC bus voltage even in the absence of the energy source, and hence avoid exposing the system to potentially damaging high voltages.
Generally speaking, the present invention fulfills the foregoing needs by providing in one exemplary embodiment thereof a method for controlling a permanent magnet machine driven by an inverter. The method allows for monitoring a signal indicative of a fault condition. The method further allows for generating during the fault condition a respective signal configured to maintain an appropriate field-weakening current that enables machine control and keeps circuitry in the motor-inverter system in a safe condition even though electrical power from an energy source is absent during said fault condition.
In another aspect of the present invention, a control system for controlling a permanent magnet machine driven by an inverter is provided. The system includes a monitor coupled to receive a signal indicative of a fault condition. The system further includes a processor coupled to supply during the fault condition a respective signal configured to maintain an appropriate field-weakening current during said fault condition.